When the air-fuel mixture supplied to an engine is adjusted by means of a carburetor, as with an automobile engine, the air-fuel ratio is adjusted by controlling the idle fuel flow rate, and also, in some cases, by controlling the bleed air flow rate. However, in carburetors used for all-purpose engines, the air-fuel ratio is adjusted by controlling the flow rates of both the main fuel and the low-speed fuel.
In order to enable such adjustments to be performed manually by manufacturers of carburetors, engines, or machines and vehicles in which carburetor-equipped engines are mounted, manual adjustment valves are included in the design of some carburetors. Such adjustment valves typically comprise a head part and a needle-shaped valve body. The head part protrudes to the outside of the carburetor main body and is used to rotate a screw rod which is screwed into the main body of the carburetor and which moves back and forth when it is rotated. The needle-shaped valve body is attached to the screw rod, and inserted into a fuel passage or bleed air passage to vary the effective area of the passage in a continuous manner.
The adjustment valve and the main body of the carburetor are both made of metal. As a result of the machining tolerance, a gap is formed between the female screw-threads in the screw hole of the carburetor main body, and the male screw-threads on the screw rod of the adjustment valve. In an attempt to prevent rattling of the adjustment valve due to the gap, a compression coil spring is installed around the screw rod between the carburetor main body and the head part.
When conventional means such as a compression coil spring are used to fasten the adjustment valve in place, the compression coil spring tends to throw the flow rate out of adjustment. This is because the compression coil spring draws the adjustment valve back by a distance equal to the aforementioned gap after the adjustment valve has been screwed into a desired position, and a screwdriver engaged with the head part is removed. The effect on the air-fuel ratio is especially great in small carburetors which are used for all-purpose engines, because the passage diameters are extremely small in such carburetors. Furthermore, because the system is designed to prevent rotation of the adjustment valve by contact friction between the compression coil spring and the head part, it is necessary to use a fairly long spring and to press the spring against the head part with a strong force to achieve an adequate rotation-checking friction. As a result, the screw rod and head part protrude a considerable distance to the outside of the main body of the carburetor. With long protruding head parts, a large rotational moment is thus generated due to vibrations of the engine, machine, or vehicle involved, causing the adjustment valve to rotate and throw the air-fuel ratio further out of adjustment. Moreover, when the carburetor is enclosed in a housing and attached to an all-purpose engine, a large housing must be provided to accommodate the long protruding head parts.
A countermeasure to unwanted rotation of the adjustment valves has been disclosed in Japanese Patent Application Kokoku No. Hei 1-28220. The disclosed arrangement includes a threadless hole formed in a retaining plate made of a synthetic resin which is an elastic material. The screw rod is passed through the threadless hole, cutting screw threads in the hole as it passes through the hole. The screw rod is then screwed into the screw hole formed in the main body of the carburetor. Thus, rotation of the adjustment valve is prevented by a plate-shaped tightening member instead of by a compression coil spring.
In the rotation-checking means described in Kokoku No. Hei 1-28220, a square tightening member is inserted into a thin square recess formed in the main body of the carburetor and cut across the screw hole. The tightening member has projections on both the front and back surfaces, and on all its outside edges. The tightening member is fastened inside the recess by the pressing contact of the projections with the inside facing surfaces and inside edge surfaces, on three sides, of the recess.
More particularly, the projections on both surfaces of the tightening member act to hold the tightening member substantially perpendicular to the axial line of the adjustment valve, while the projections on the outside edges act to hold the tightening member so that the tightening member cannot rotate. However, equipment such as a special mold, etc., is required to form such a tightening member with projections, complicating the manufacture of the tightening member.
In addition, because the screw rod of the adjustment valve cuts screw threads as it passes through the hole of the tightening member, the elastic force of the synthetic resin material of the tightening member cannot act sufficiently on the screw rod. As a result, the adjustment valve tends not to be fastened in a strong and stable manner.
Thus, even where a plate-shaped tightening member is used instead of a compression coil spring as a means of checking the rotation of the adjustment valve, and also as a countermeasure to the adjustment valve drawback problem and the need to lengthen the adjustment valve's protruding head portion, problems remain because it is difficult to mold the plate-shaped tightening member, and the member is not very reliable as a rotation-checking member.